1. Field of the Invention
The invention is related to the field of flow meters, and in particular, to Coriolis flow meters.
2. Description of the Prior Art
Coriolis flow meters typically operate by vibrating one or more flow tubes and measuring deflections, or phase differences, in the vibrating flow tubes induced by the Coriolis forces from a material flowing through the flow tubes. Coriolis flow meters have a number of different flow tube designs. Some meters have straight flow tubes and some have curved flow tubes. Some meters have a single flow tube and some have two flow tubes. Each type of Coriolis flow meter has been developed to address different problems in the operation of the flow meter. One of the problems addressed has been the vibration of the flow meter at the connecting point to the piping system. Typically the flow meter will have a flange at each end of the meter to allow the meter to be coupled into the piping system.
Dual tube designs typically split the flow of material into two streams using manifolds and send the two streams of material into the two flow tubes. Because the flow is split into two streams, the diameter of the flow tubes need not be the same as the diameter of the piping system. The two tubes are typically symmetrical in shape and mounted parallel to one-another. The two tubes typically vibrate at the same frequency but in the opposite phase. Because the tubes are symmetrical and vibrated opposite each other, the vibrations typically cancel out where the two tubes are joined. This creates a balanced flow meter (i.e. little or no vibration of the meter at the manifolds). A change in density in the material flowing through the two tubes changes the mass of both tubes equally and therefore the two tube designs remain balanced across a wide range of material densities. The two tubes are typically joined together at the manifolds. Splitting a wide range of different materials into two equal flows is a difficult task for a dual tube design. Splitting the flow can also create a greater pressure drop across the flow meter. In addition, material can become clogged at the split point inside the manifold.
Single tube designs don't split the flow into two streams. This eliminates the problems associated with splitting the flow into two equal streams. Because there is only a single vibrating tube, another method must be used to eliminate the vibration of the flow meter at the flanges. Straight single tube designs may use a counterbalance member surrounding at least a portion of the vibrating flow tube. One such meter is disclosed in U.S. Pat. No. 6,401,548 “Coriolis mass flow/density sensor”. Curved single flow tube designs have used a number of techniques to eliminate the vibration of the meter at the manifolds. One technique is to include a support plate having a mass substantially higher than the mass of the vibrating tube, for example U.S. Pat. No. 5,705,754 “Coriolis-Type mass flowmeter with a single measuring tube”. Another technique is to have two tubes parallel to each other, but only flow material through one of the tubes. The second “dummy” tube is used as the counter balance and vibrates in opposite phase with the measuring tube. An example of this technique is show in U.S. Pat. No. 6,666,098 “Vibratory transducer”. Another technique is to build a structure attached to the single tube that has a member that vibrates in the opposite phase of the vibrating tube, for example U.S. Pat. No. 6,484,591 “Mass flow rate/density sensor with a single curved measuring tube”. These methods may create a balanced meter for a single material at a given density. Unfortunately, when the density of the material changes or a different material with a different density is measured, the flow meter is typically no longer in balance.
Therefore there is a need for a system and method for balancing a single curved tube Coriolis flow meter over a range of material densities.